you can!!!
There is no limit.
There may be conflict of interests, though. You can't be a theist and an atheist at the same time, for example.
Of course you can be in theist and atheist group.Real theists love everyone.If I was a theist , an atheist group would be very intriguing to be.The hypocrit theists are the ones that have the problem.
Well, is there? I want to be in them all!